The invention relates to an analog power amplifier (PA), particularly for a closed loop signal amplification requiring high output power, high speed and wide operating temperature range.
Power amplifiers are devices designed to amplify an input signal and provide an undistorted high power output signal, i.e. an alternate current within a wide range of an output voltage; independent of supply voltages, load fluctuations over frequency, operating temperature, etc. The parameters such as input impedance, input offset voltage or open loop gain are not essential.
Conventional PAs comprise single-ended stages which results in a low and asymmetrical slew rate and a slow recovery from clipping. Multiple stages cause cumulative phase shift which impairs the stability. Crossover distortions are minimized by excessive quiescent current causing an extensive power dissipation even with no load. The switching of the power transistors is not completely eliminated however, as each transistor is cut off at a higher output current of the complementary transistor, even worse, a reverse base-emitter voltage is applied thereto.
A thermal compensation, simple in principle, is very inaccurate, unreliable and difficult to accomplish; it demands temperature compensation of at least two complementary high power transistors by means of floating low power components thermally coupled thereto. The quiescent current must be adjusted manually, whereby its stability is very poor. A matching of power transistor is laborious and troublesome, and in case of MOSFETs very difficult to achieve. Furthermore, the temperature coefficient of the MOSFET's gate-source voltage varies in a very wide range and has a zero value at a drain current mostly much higher than a desired quiescent current. Collectors or drains of the transistors, coupled to the cases thereof during a manufacturing process, are on different potentials. Insulating wafers introduce large parasitic capacitances and increase thermal impedance keeping individual devices at more uneven temperature.
The output voltage swing is reduced by the base-emitter or gate-source voltages of the power transistors which can be significant for high output currents and is generally very high for power MOSFETs. In order to improve the stability and accuracy of the quiescent current and minimize the likehood of thermal runaway of the power transistors, power resistors coupled is series with the load are used.
The efficiency is improved by providing a multiple level power supply. The supply voltage of the PA is switched to different values according to the output signal level, usually by means of power switches. The transition must occur at a relatively high voltage across a corresponding power transistor of the amplifier in order to prevent its saturation. The instant voltage switching at a high output current puts an enormous stress on the power transistors designed for a signal amplification, and degrades their long-term reliability. Moreover, the distortion level of the PA is significantly higher due to the switching.